7.
I agree! PMD has much more story than the main series, where it's the same global adventure every time where the player collects badges and fights the regional evil team.
This. While I can't speak for Gates to Infinity (considering I've never played it before), Explorers of Sky has the most well-written plot out of any Pokémon game I've ever played, and is
easily better than the main series games in that regard. The story utilizes a stunning amount of depth, background, plot setup, and character development which all add up to a very compelling story, despite seeming overly light-hearted at the beginning.
I'll admit that the Rescue Team games had a pretty simplistic story, but they were still likable nonetheless. More importantly, they have nothing to do with the Explorers games, so you won't be missing anything if you start out with Explorers of Sky. (Well, other than musical references, haha.)
what's your favorite spin-off game?
That would have to be Explorers of Sky for me, haha. However, Explorers of Time/Darkness aren't necessarily obsolete, because they're
technically more tragic, if you know what I mean.